1. Technical Field
The present invention relates to a step-up switching power supply wherein the turning on and off of a switching element is fixed directly from the result of comparing a detected output voltage value and a reference voltage, without using an error amplifier.
2. Related Art
Heretofore known switching power supplies are mainly such that the difference between a detected output voltage value and a reference voltage is amplified by an error amplifier, thereby generating an error signal, and the turning on and off of a switching element is controlled by the error signal. An example of a configuration of a heretofore known step-up switching power supply using an error amplifier is shown in FIG. 4.
The step-up switching power supply shown in FIG. 4 includes voltage divider resistors R1 and R2 (resistance values thereof are also expressed as R1 and R2), an error amplifier ErrAmp, a comparator Comp1, an RS flip-flop FF1, an oscillator circuit OSC, a drive circuit Dry, an inductor L, a switching element Msw formed of an N-channel MOS transistor, a current detector circuit Idet, a rectifying element, which may be a diode D, and an output capacitor Cout. An output voltage Vout is the voltage across the output capacitor Cout, and is divided by the voltage divider resistors R1 and R2, thus providing a detected value (feedback voltage) Vfb of the output voltage Vout. The error amplifier ErrAmp amplifies the difference between the detected value (feedback voltage) Vfb and a reference voltage Vref, and outputs an error signal Ve. The comparator Comp1 compares the error signal Ve and an output voltage Videt of the current detector circuit Idet, which represents the value of the current flowing through the switching element Msw, and resets the RS flip-flop FF1 when the output voltage Videt of the current detector circuit Idet reaches the error signal Ve (when Videt≧Ve). Meanwhile, the oscillator circuit OSC provides a set pulse of a constant cycle to the RS flip-flop FF1. The RS flip-flop FF1, via the drive circuit Drv, controls the turning on and off of the switching element Msw so that the switching element Msw is turned on when the RS flip-flop FF1 is set, and turned off when the RS flip-flop FF1 is reset.
On the switching element Msw being turned on, the inductor L is connected between an input voltage Vin (the high potential side potential of an input power source) and a reference potential GND (the low potential side potential of the input power source), thereby increasing the current flowing through the inductor L. On the switching element Msw being turned off, the current flowing through the inductor L commutates to the diode D, and charges the output capacitor Cout. When the current of the inductor L flows to the diode D, the current flowing through the inductor L decreases. When the switching element Msw is next turned on, the current of the inductor L commutates from the diode D to the switching element Msw.
As an operation of the switching power supply, an operation whereby the current flowing through the inductor L increases, and energy is stored in the inductor L, when the switching element Msw is in an on-state, the switching element Msw is turned off when the current flowing through the switching element Msw reaches a value corresponding to the error signal Ve, and the energy stored in the inductor L is discharged to the output capacitor Cout, is repeated in every cycle of the oscillator circuit OSC, and the output voltage Vout is controlled to the voltage indicated by the reference voltage Vref.
The error amplifier ErrAmp is a functional element for carrying out a negative feedback control whereby the time for which the switching element Msw is in an on-state is shortened by the error signal Ve being lowered when the detected value (divided value) Vfb of the output voltage Vout is higher than the reference voltage Vref, and the time for which the switching element Msw is in an on-state is lengthened by the error signal Ve being raised when Vfb is lower than the reference voltage Vref, and the output voltage Vout is controlled to Vout=Vref×(R1+R2)/R2 by the negative feedback control. Herein, in order that the negative feedback control is carried out stably, it is necessary for phase compensation of the error amplifier ErrAmp to be appropriately carried out.
However, responsiveness deteriorates when phase compensation is carried out for the error amplifier ErrAmp, and there is a problem in that response to a sudden change in the load, or the like, is delayed. Also, as a phase compensation circuit, the oscillator circuit OSC, or the like, is necessary, there is also a problem in that the switching power supply control circuit becomes complex.
In response to this, a control method called hysteresis control or ripple control has been proposed (for example, refer to JP-A-2006-14559, JP-A-2007-159395, and JP-A-2011-205743).
A configuration of a heretofore known switching power supply using hysteresis control is shown in FIG. 5. FIG. 5 is a step-down switching power supply including the voltage divider resistors R1 and R2, comparators Comp2 and Comp3, an RS flip-flop FF2, the oscillator circuit OSC, the drive circuit Drv, the inductor L, the switching element Msw formed of an N-channel MOS transistor, the diode D, and the output capacitor Cout. The output voltage Vout is the voltage across the output capacitor Cout, and is divided by the voltage divider resistors R1 and R2, thus providing the detected value (feedback voltage) Vfb of the output voltage Vout. The comparators Comp2 and Comp3 compare the detected value Vfb of the output voltage Vout with reference voltages Vref10 and Vref20 (Vref10<Vref20) respectively, the comparator Comp2 sets the RS flip-flop FF2 when the detected value Vfb of the output voltage Vout becomes equal to or lower than the reference voltage Vref10, and the comparator Comp3 resets the RS flip-flop FF2 when the detected value Vfb of the output voltage Vout becomes equal to or higher than the reference voltage Vref20. The RS flip-flop FF2, via the drive circuit Dry, controls the turning on and off of the switching element Msw so that the switching element Msw is turned on when the RS flip-flop FF2 is set, and turned off when the RS flip-flop FF2 is reset.
As an operation of the switching power supply, current is supplied from the input voltage Vin to the load via the inductor L when the switching element Msw is in an on-state, and current flowing through the inductor L increases, whereby energy is stored in the inductor L, and on the switching element Msw being turned off, the current flowing through the inductor L commutates, current flows into the inductor L from the reference potential GND (the low potential side potential of the input power supply) rather than from the input voltage Vin, and the current flowing through the inductor L, that is, the energy of the inductor L, decreases. By the heretofore described operation being repeated, the output voltage Vout is controlled to the voltages indicated by the reference voltages Vref10 and Vref20. That is, the output voltage Vout has a ripple waveform that rises and falls between Vref10×(R1+R2)/R2 and Vref20×(R1+R2)/R2 (the name ripple control has its origin here).
As this method does not involve phase compensation, high-speed response is possible, and as no oscillator circuit is necessary either, it is also possible to achieve a simplification of the control circuit.
However, the hysteresis control type switching power supplies disclosed in the patent documents are all step-down types, with no step-up type. It is supposed that this depends on whether or not the timing at which the switching element is turned off can be easily determined. That is, it is because, when considering the output voltage in an on-state period for which the switching element is in an on-state, the step-down switching power supply is such that the input voltage and output terminal are connected via the inductor L, and the output voltage rises, because of which the timing at which the switching element is turned off using the circuit shown in FIG. 5 can be easily determined, whereas a step-up switching power supply is such that the current flowing through the inductor L flows to the switching element Msw and not to the output terminal, because of which the output voltage does not rise, and it is not possible as things stand to determine the timing at which the switching element is turned off.